In some modernized internal combustion engines for automotive vehicles, the exhaust gases once discharged from the power cylinders into the exhaust system of the engine are partially recirculated at controlled rates into the air-fuel mixture system of the engine for thereby controlling the exhaust gas emission. An exhaust gas recirculation system which has been used for this purpose in an internal combustion engine for an automotive vehicle includes a diaphragm-operated exhaust gas recirculation rate control valve adapted to vary the recirculation rate of exhaust gases depending upon the induction rate of air through the mixture supply system of the engine. The induction rate of air through the mixture supply system is usually detected from a vacuum developed in the mixture supply system when the internal combustion engine is in operation.
In an advanced version of a prior-art exhaust gas recirculation system of this nature, the recirculation rate control valve is used in combination with a diaphragm-operated vacuum compensating valve responsive to the pressure of the exhaust gases to be passed through the recirculation rate control valve. The vacuum compensating valve is adapted to supply atmospheric air into the vacuum chamber in the recirculation rate control valve at rates variable with the pressure of the exhaust gases to be passed through the recirculation rate control valve so that the vacuum to act on the diaphragm of the recirculation rate control valve is lessened or compensated on the basis of the pressure of the exhaust gases to be passed through the recirculation rate control valve.
The exhaust gases recirculated from the exhaust system are directed to the recirculation rate control valve through a flow restriction or orifice provided upstream of the valve. When the recirculation rate control valve is in a fully open or nearly fully open condition, the exhaust gas pressure developed between the orifice and the recirculation rate control valve is reduced toward an atmospheric or even subatmospheric level. The reduced exhaust gas pressure or the vacuum thus developed downstream of the orifice acts on the diaphragm of the vacuum compensating valve and causes the valve to supply atmospheric air or increase the supply rate of air to the vacuum chamber of the recirculation rate control valve. This causes the recirculation rate control valve to interrupt the recirculation of exhaust gases or reduce the recirculation rate of exhaust gases therethrough.
As the pressure of the exhaust gases in the exhaust system of the engine increases with an increase in the load on the engine, the exhaust gas pressure developed between the orifice and the recirculation rate control valve and acting on the diaphragm of the vacuum compensating valve is also increased and holds the diaphragm of the vacuum compensating valve in a position interrupting the supply of air to the vacuum chamber of the recirculation rate control valve. Under these conditions, the recirculation rate control valve is operative to control the exhaust gas recirculation rate in accordance with the relationship between the pressure of the exhaust gases passed through the orifice and the vacuum developed in the mixture supply system.
In order to permit exhaust gases to be recirculated at satisfactory rates under conditions in which relatively low exhaust gas pressure occurs upstream of the orifice, it is important that the vacuum compensating valve be designed to be responsive to exhaust gas pressures close to an atmospheric level. For this reason and further by reason of the pulsation in the pressure of the exhaust gases, the exhaust gas recirculation rate achieved is subject to fluctuations and, therefore, can not be controlled accurately insofar as only the pressure of the exhaust gases to be recirculated is utilized as the control signal for the vacuum compensating valve.
Since, furthermore, the exhaust gas recirculation rate is controlled to vary with the exhaust gas pressure proportional to the induction rate of air through the mixture supply system of the engine, the resultant recirculation rate is inevitably maintained in the vicinity of a certain fixed value for different modes of operation of the engine, causing reduction in the total performance efficiency of the engine under various operating conditions thereof. It may also be pointed out that meticulous design considerations and high production accuracy are required for the design and engineering of the vacuum compensating valve which is required to be responsive to extremely low exhaust gas pressures as above discussed.
The present invention contemplates elimination of these drawbacks which have been inherent in a prior-art exhaust recirculation system of the described nature.